The present invention relates to a plasma display device, a luminance correction method and a display method thereof adapted for carrying out luminance correction in the plasma display device where display is performed by utilizing AC plasma discharge.
A plasma display panel (PDP) is adapted for constituting a thin structure with a great screen, and future development is expected particularly in realizing large-size display devices.
The plasma display panel of such a device is composed of two glass substrates opposed to each other and joined together with a discharge gas sealed therein. A pair of parallel sustain electrodes are disposed on the front glass substrate, and an address electrode is disposed on the back glass substrate in a direction to intersect with the sustain electrodes. The inside of one substrate is coated with a phospher layer. When a predetermined voltage is applied to the sustain electrodes, plasma discharge is generated between the paired electrodes to radiate ultraviolet rays, which are then incident upon the phospher layer to emit light therefrom. FIG. 15 is a schematic diagram showing an electrode structure on a display panel where pixels of m×n dots are provided. There are arranged n sets (X1, Y1, X2, Y2, . . . , Xn, Yn) of paired sustain electrodes 107X, 107Y, and m sets (A1, A2, . . . , Am) of address electrodes 103A, wherein the paired sustain electrodes 107 intersect with the address electrodes 103A to constitute a matrix in which a pixel is positioned at each intersection, as indicated by dotted lines in this diagram.
Emission of light per pixel is normally controlled at three steps, and respective operation periods are termed a reset period, an address period and a (discharge) sustain period. In a selective erase system for example, voltages of the waveforms shown in FIGS. 16A to 16C are applied, during the individual operation periods, to the three electrodes constituting each pixel. During the reset period, the entire sustain electrodes 107X and 107Y are discharged and the wall charges in the entire pixel regions are stored uniformly, so that the data stored previously in the pixels are wholly erased and the entire screen is kept in an even charged state. In the next address period, a binary state is formed depending on the presence or absence of the wall charge, and the pixel to be driven for emission of light is selected. At this time, addressing is executed in the following procedure with the sustain electrodes 107Y (Y1, Y2, . . . , Yn) being used as scanning electrodes and the address electrodes 103A as data electrodes, respectively.
Pulses are inputted sequentially to the sustain electrodes 107Y (Y1, Y2, . . . , Yn) at predetermined timings, and simultaneously data pulses corresponding to emission/non-emission of light from the pixels selected according to the combination with the voltage-applied sustain electrodes 107Y (in this case, relative to the non-emission pixels) are inputted to the m sets of the entire address electrodes 103A (A1, A2, . . . , Am) synchronously with the scanning timing on the sustain electrodes 107Y side. As a result, a discharge is generated in the non-emission pixel, and the wall charge is erased. Subsequently in the sustain period, an AC pulse voltage (sustain pulse) is applied to the paired sustain electrodes of the entire pixels. At this time, only the pixels having a residual wall charge reach a discharge start voltage selectively, and the generated discharge is sustained so that the light is emitted continuously during this period.
In this manner, the plasma display panel (PDP) executes display by emission of light under digital control. Generally, a sub-field method is employed as a driving system. The sub-field method is carried out by time-dividing one field of the display screen into some sub-fields and displaying the brightness gradations through time-width modulation of the light emission time. According to this method, the one-field display period (16.7 msec) is weighted in proportion to the bit place of N-bit image data, and is divided into N sub-fields where the light is emitted 2 k times (where k=0 to N−1) respectively. For example, if the image data per pixel are composed of 8 bits, the 1-field display period is divided into sub-fields SF1-SF8, and the number of times of light emission during the sub-fields SF1-SF8 is set sequentially to 20(1), 21(2), 22(4), . . . , 27(128). The emission of light can be performed 0 to 255 times by combining the on/off actions in such eight sub-fields, hence realizing display in 256 gradations.
This sub-field method is premised on that the luminance level at the time of light emission is kept always constant. Actually, however, in a display region where “ON” display pixels occupy a large area, a voltage drop is derived from the output impedance of a driving IC or from the wiring resistance of the display panel and so forth, whereby the luminance level at the time of light emission is reduced correspondingly to the drop of the supply voltage. For example, in case the regions being displayed brightly in the image are collected together to be more than certain dimensions, there exists a problem that such regions fail to be displayed at the desired brightness.
Another problem is to secure proper gradations in displaying a dark image. FIG. 18 graphically shows a typical video signal prior to being converted into image data. In the video signal, the luminance is expressed by an amplitude where a white peak level (white level) is maximum and a blanking level (black level) is minimum. Normally this signal is quantized to become image data in such a manner that 8 bits are allocated to the full range from a white level to a black level, whereby the full range luminance is expressed in 256 gradations. However, when a wholly dark image is to be displayed, the luminance differences of the entire screen are expressed by, e.g., 3 Low-order bits or so which correspond substantially to 8 gradations. In this case, since the original video signal is analog, the darkness is rendered homogeneous due to shortage of the number of gradations although infinitely fine luminance difference information is contained therein, whereby the luminance differences cannot be discriminated to consequently fail in attaining a desired screen quality.